In the continuous casting steel-making process, molten steel is poured from a ladle into a large vessel known as a tundish via a ladle shroud. The tundish has one or more outlets through which the molten steel flows from the tundish into one or more respective moulds. The molten steel cools and forms a solid skin in the moulds and eventually forms continuous solid strands of metal. A submerged entry nozzle or casting nozzle is located between the tundish and each mould to control the flow characteristics of the molten steel flowing from the tundish to the mould and prevent the ingress of air. The rate of steel flow into each mould is often controlled by a stopper rod which resides in the tundish and can be moved vertically by a lifting apparatus into and out of the inlet of the submerged entry nozzle.
Many of the refractory bodies, such as the ladle shroud, submerged entry nozzle and stopper rod, have regions that come into frequent contact with a layer of slag that settles on top of the molten metal. The slag is highly corrosive and thus all of the aforementioned devices are at risk of corrosion after being submerged or partially submerged in the molten metal for relatively short periods of time unless they are protected in some manner from the corrosive properties of the slag.
A common solution to this problem is to provide a “slag band” i.e. a wear resistant zone of material in the region of the refractory body that is likely to contact the slag in use. One such material is carbon-bonded zirconia-graphite. However, its use is hindered by the fact that zirconia is polymorphic, existing in a monoclinic form at room temperature, changing to a tetrahedral structure at 1170° C. and a cubic form at about 2300° C. The monoclinic to tetrahedral change is accompanied by a reversible volume change (shrinkage) of about 5% (see FIG. 1) which leads to cracking of the grains and hence failure of the refractory. This undesirable volumetric change has been alleviated to some extent by the addition of controlled quantities of various cubic oxides, such as calcia, magnesia and yttria. These stabilising oxides form a solid solution with the zirconia and give rise to a structure which is a mixture of cubic and monoclinic zirconias, known as ‘partially stabilised zirconia’ (PSZ). PSZ is utilized in slag bands as it is considered to exhibit the optimum balance of thermal expansion and thermal shock resistance properties.
A drawback associated with the use of PSZ for slag bands is that the high thermal expansion coefficient of the material (10×10−6/° C.) necessitates pre-heating of the refractory before it can be used for the flow of liquid steel. Pre-heat temperatures are normally in the range 900° C. to 1400° C. and pre-heat times are usually between 1 to 8 hours. This is clearly undesirable as it increases the cost of the process and causes a lengthy downtime if the casting process has to be stopped for any reason. Steel manufacturers require cold start capabilities from slag bands for submerged entry nozzles/shrouds in particular, in emergencies such as when a strand is lost because of failure to start. In order to maintain casting of steel in such circumstances, an un-preheated tube is put into service on a strand held in reserve. These cold start-up tubes may be provided with a slag band manufactured with approximately 10% of the zirconia replaced by silicon carbide and the tubes are decarburised. However, whilst the low thermal expansion of silicon carbide confers sufficient thermal shock resistance for a cold start, the silicon carbide is soluble in the mould slag. Hence, this serves only as a temporary measure as the corrosion resistance of the tube is seriously compromised.